1. Field of the Invention
The present invention relates to a method of preventing corrosion of metal. More particularly, the method comprises applying a solution containing one or more multi-functional silanes to a metal substrate. If the metal is to be painted, this treatment may be followed by applying a solution of an organofunctional silane. The method is particularly useful for the treatment of cold-rolled steel, aluminum and aluminum alloys.
2. Description of Related Art
Most metals are susceptible to corrosion, in particular atmospheric corrosion. Such corrosion will significantly affect the quality of such metals, as well as that of the products produced therefrom. Although this corrosion may sometimes be removed from the metal, such steps are costly and may further diminish the utility of the final product. In addition, when polymer coatings such as paints, adhesives, or rubbers are applied to the metal, corrosion of the base metal material may cause a loss of adhesion between the polymer coating and the base metal. A loss of adhesion between the polymer coating and the base metal can likewise lead to corrosion of the metal. Aluminum alloys are particularly susceptible to corrosion as the alloying elements used to improve the metals mechanical properties (e.g., copper, magnesium and zinc) will decrease corrosion resistance.
Prior art techniques for improving corrosion resistance of metal, particularly metal sheet, include passivating the surface by means of a heavy chromate treatment. Such treatment methods are undesirable, however, because the chromium is highly toxic, carcinogenic and environmentally undesirable. It is also known to employ a phosphate conversion coating in conjunction with a chromate rinse in order to improve paint adherence and provide corrosion protection. It is believed that the chromate rinse covers the pores in the phosphate coating, thereby improving the corrosion resistance and adhesion performance. Once again, however, it is highly desirable to eliminate the use of chromate altogether. Unfortunately, the phosphate conversion coating is generally not effective without the chromate rinse.
Recently various techniques for eliminating the use of chromate have been proposed. These include the steps of providing an aqueous alkaline solution comprising an inorganic silicate and a metal salt in an amount to coat a steel sheet, followed by treating the silicate coating with an organofunctional silane (U.S. Pat. No. 5,108,793). U.S. Pat. No. 5,292,549 teaches the rinsing of metal sheet with an aqueous solution containing low concentrations of an organofunctional silane and a crosslinking agent, such as bistrimethoxy silylethane in order to provide temporary corrosion protection. The crosslinking agent cross links the organofunctional silane to form a denser siloxane film. In one embodiment an organofunctional silane solution is used to treat a metal surface followed by application of a second treatment solution 3 containing a crosslinking agent. One significant drawback to the methods of this patent is that the organofunctional silane will not bond well to the metal surface, and thus the coating may be easily rinsed off.
U.S. Pat. No. 5,393,353 describes a method for preparing a chromium-free black zinc-nickel alloy surface containing at least about 8% nickel in the alloy. The method comprising contacting said surfaces with an aqueous acid solution of an inorganic acid, preferably a phosphorus acid, the solution optionally additionally containing at least one silane, such as bistrimethoxysilylethane.
U.S. Pat. No. 4,689,085 describes coupling agent and primer compositions comprising a conventional silane couplying agent and a disilyl cross linker compound in the same composition solution.
U.S. Pat. No. 5,433,976 teaches the rinsing of a metal sheet with an alkaline solution containing a dissolved silicate or aluminate, an organofunctional silane and a cross-linking agent in order to form an insoluble composite layer containing siloxane.
The water solubility of some silanes may be limited creating solubility problems in the aqueous solution. Additionally, some metal substrates are particularly prone to corrosion upon contact with water, thus, an aqueous solution may be undesirable in some circumstances.
Various other techniques for preventing the corrosion of metal sheets have also been proposed. Many of these proposed techniques, however are ineffective, or require time-consuming, energy-inefficient, multi-step processes. Thus, there is a need for a simple, low-cost technique for preventing corrosion of metals, particularly metal sheet and cast aluminium or aluminium alloys, including those instances wherein a polymer coating such as paint is to be applied to the metal.